1. Field of the Invention
The invention relates to the production of nitrogen from air. More particularly, it relates to the production of high purity nitrogen.
2.Description of the Prior Art
Permeable membrane processes and systems have been increasingly employed in air separation operations for the production of nitrogen. In such operations, feed air is brought into contact with the surface of the membrane, and oxygen, as the more readily permeable component of air, passes through the membrane while nitrogen, the less readily permeable component of air, is withdrawn from the membrane system as a nonpermeable product stream.
Although the fundamentals of gas separation using membranes have been known for a long time, it was not until recently that advances in membrane fabrication and packaging techniques have made membrane technology economically attractive for commercial air and other gas separations. Because of such developments and the inherent simplicity of the membrane technology, a high level of interest and activity exists with respect to gas separations in the membrane art, notably in the field of air separation applications.
Single stage hollow fiber membrane processes and systems have been developed for the production of enriched nitrogen from air. This approach has the advantage of minimizing the capital costs associated with membrane staging, fabrication, piping and the like. As the desired nitrogen purity level increases, however, product recovery decreases and the power and membrane surface area requirements increase, thereby rendering single stage operation less desirable from an overall viewpoint.
For nitrogen product purities above about 94%, two stage membrane processes and systems are desirable as an alternative to single stage operation. In two stage operations, with oxygen as the more selectively permeable component of feed air and nitrogen as the less selectively permeable component thereof, the permeate gas from the second stage is typically recycled. The blending of the permeate gas, which is nitrogen-rich as compared to air, with the feed air to membrane system reduces the oxygen content of the feed to the system and enhances nitrogen recovery over the obtainable using a single membrane stage. In such two-stage membrane operations, no extra machinery is required since the low pressure permeate recycle from the second stage is returned to the suction side of the feed gas compressor.
Two stage membrane systems are commonly employed to produce nitrogen product at purity levels of from about 97% to about 99.9%, with 98% nitrogen product being a typical product of such membrane operations. At high nitrogen purities above 99.0-99.7%, however, two stage membrane systems tend to become quite expensive. Thus, more power and increased membrane surface area are required to produce such high purity levels at given membrane permeation pressures. Alternatively, more power and increased trans-membrane pressure are required to produce such high purity nitrogen for a given surface area membrane. While two stage operations can be employed to produce nitrogen product at 99.99+% purity levels, as can single stage systems, the overall technical and economic feasibility of employing such one or two stage systems are diminished by the high costs of such operations at said high purity levels.
In order to achieve very high purity nitrogen product, e.g., above about 99.5%, by the highly desirable membrane approach, a two stage air separation membrane system has been integrated with a deoxo unit, in which residual oxygen in the nitrogen stream removed from the air separation membrane system is reacted with hydrogen or a fuel gas, such as methane. Such integrated membrane/deoxo systems, disclosed and illustrated in Prasad, U.S. Pat. No. 4,931,070, can be used to produce nitrogen product having a purity of up to about 99.95% or even higher, such as ultra-high purity levels on the order of about 99.999%. While such integrated two stage membrane/deoxo systems enable very high purity nitrogen product, including ultra-high purity nitrogen, to be achieved in a manner not feasible using the prior art one and two stage membrane systems referred to above, further improvement in the art is desirable in order to enable such increasingly high nitrogen purity requirements to be met on a more economically feasible basis, or without the use of hydrogen or other fuel gases.
In light of such industry requirements and expectations for the highly advantageous membrane technology approach to air separation, as well as other gas separations, attention has been directed to three stage membrane systems as an alternative to the use of a deoxo unit with two stage systems. In this regard, it is noted that three or more membrane stages have been employed heretofore in the so-called cascade separation approach to achieve enrichment of the permeate component of a feed gas mixture. For this purpose, the permeate gas separated from each membrane stage is passed as feed gas to the next succeeding membrane stage with an enriched permeate gas, e.g., oxygen in the case of air separation, being recovered from the last membrane stage. Non-permeate gas, e.g., nitrogen is removed from each such stage. This approach is not directed to the achieving of enhanced purity levels of the non-permeate gas.
The use of three membrane stages in air separation for very high nitrogen purity production is disclosed in "Nitrogen Production Using Membranes", Thompson, Prasad, Gottzmann and Reul-Heeren, a paper presented at a symposium at Antwerp, Belgium, Sep. 10-15, 1989.
FIG. 1 of said paper illustrated one, two and three stage membrane systems for the recovery of nitrogen by air separation. In the three stage system illustrated therein, feed air is passed from a feed compressor to a first stage membrane from which a more selectively permeable oxygen stream is discharged to waste, with the less permeable nitrogen stream separated therefrom being passed to the second stage. The permeate stream from said second stage is recycled for compression with additional quantities of feed air being passed to the membrane system. The second stage non-permeate gas is passed to the third stage membrane, from which very high purity nitrogen product is recovered as non-permeate gas. The oxygen containing permeate gas from the third stage is compressed and recycled for passage to the second stage membrane together with additional quantities of the first stage permeate gas.
The three stage membrane system provides a potentially desirable alternative to the use of two membrane stages, together with a deoxo unit, for the production of nitrogen at high and very high purity levels, except for production of nitrogen at ultra-high purity levels. It will be appreciated that the desirable recycle of third stage permeate gas to the inlet to the second stage membrane requires the use of an additional compressor to boost the third stage permeate gas to the desired permeation pressure level for recycle of said gas to the second stage membrane. As those skilled in the art will readily appreciate, the benefits derived from the use of the additional third stage recycle, such as higher product recovery, less membrane area and the like, must outweigh the capital and operating costs associated with providing such third stage recycle feature such as the additional compressor. There is a genuine need and desire in the art to achieve such additional benefits in an economical manner so that the inherent simplicity and advantages of the membrane approach can be further extended to the production of high purity nitrogen from air without the need for combining the membrane system employed with a deoxo unit or any other such means for achieving such high purity levels.
Very high purity nitrogen is produced by air separation in tile membrane system of three or more stages disclosed by the Prasad patent, U.S. Pat. No. 5,102,432. In this system, the third stage permeate is recycled to the second stage, and the membrane surface area is distributed between the stages to achieve high product recovery and process performance. While the process and system of said Prasad patent represents a highly desirable advance in the art, further improvement is nevertheless desired in order to reduce the power requirements of multi-stage membranes. In particular, there are mixing losses associated with the blending of nitrogen-rich recycle streams with feed streams of a previous stage. The recycle streams are generally richer in nitrogen than are the retentate, or nonpermeate, streams with which they are mixed. This mixing increases the entropy and decreases the overall efficiency of the air separation operation. The elimination of such mixing losses would result in a more efficient and economical process for the production of very high purity nitrogen.
It is an object of the invention, therefore, to provide an improved membrane process and system for the production of nitrogen at high purity levels from air.
It is another object of the invention to provide an improved process or system utilizing at least three membrane stages for the production of high and very high purity nitrogen by air separation.
It is a further object of the invention to provide an improved membrane process and system for the separation of air and the production of high and very high purity nitrogen without the need for employing a deoxo unit therewith.
With these and other objects in mind, the invention is hereinafter described in detail, the novel features thereof being particularly pointed out in the appended claims.